//: Routines can be put in a 'waiting' state, from which it will be ready to
//: run again when a specific memory location changes its value. This is mu's
//: basic technique for orchestrating the order in which different routines
//: operate.

:(scenario wait_for_location)
recipe f1 [
  1:number <- copy 0
  start-running f2:recipe
  wait-for-location 1:number
  # now wait for f2 to run and modify location 1 before using its value
  2:number <- copy 1:number
]
recipe f2 [
  1:number <- copy 34
]
# if we got the synchronization wrong we'd be storing 0 in location 2
+mem: storing 34 in location 2

//: define the new state that all routines can be in

:(before "End routine States")
WAITING,
:(before "End routine Fields")
// only if state == WAITING
long long int waiting_on_location;
int old_value_of_waiting_location;
:(before "End routine Constructor")
waiting_on_location = old_value_of_waiting_location = 0;

//: primitive recipe to put routines in that state

:(before "End Primitive Recipe Declarations")
WAIT_FOR_LOCATION,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["wait-for-location"] = WAIT_FOR_LOCATION;
:(before "End Primitive Recipe Implementations")
case WAIT_FOR_LOCATION: {
  reagent loc = canonize(current_instruction().ingredients.at(0));
  Current_routine->state = WAITING;
  Current_routine->waiting_on_location = loc.value;
  Current_routine->old_value_of_waiting_location = Memory[loc.value];
  trace(Primitive_recipe_depth, "run") << "waiting for location " << loc.value << " to change from " << Memory[loc.value] << end();
  break;
}

//: scheduler tweak to get routines out of that state

:(before "End Scheduler State Transitions")
for (long long int i = 0; i < SIZE(Routines); ++i) {
  if (Routines.at(i)->state != WAITING) continue;
  if (Routines.at(i)->waiting_on_location &&
      Memory[Routines.at(i)->waiting_on_location] != Routines.at(i)->old_value_of_waiting_location) {
    trace("schedule") << "waking up routine\n" << end();
    Routines.at(i)->state = RUNNING;
    Routines.at(i)->waiting_on_location = Routines.at(i)->old_value_of_waiting_location = 0;
  }
}

//: also allow waiting on a routine to stop running

:(scenario wait_for_routine)
recipe f1 [
  1:number <- copy 0
  12:number/routine <- start-running f2:recipe
  wait-for-routine 12:number/routine
  # now wait for f2 to run and modify location 1 before using its value
  3:number <- copy 1:number
]
recipe f2 [
  1:number <- copy 34
]
+schedule: f1
+run: waiting for routine 2
+schedule: f2
+schedule: waking up routine 1
+schedule: f1
# if we got the synchronization wrong we'd be storing 0 in location 3
+mem: storing 34 in location 3

:(before "End routine Fields")
// only if state == WAITING
long long int waiting_on_routine;
:(before "End routine Constructor")
waiting_on_routine = 0;

:(before "End Primitive Recipe Declarations")
WAIT_FOR_ROUTINE,
:(before "End Primitive Recipe Numbers")
Recipe_ordinal["wait-for-routine"] = WAIT_FOR_ROUTINE;
:(before "End Primitive Recipe Implementations")
case WAIT_FOR_ROUTINE: {
  if (SIZE(ingredients) != 1) {
    raise << current_recipe_name() << ": 'wait-for-routine' requires exactly one ingredient, but got " << current_instruction().to_string() << '\n' << end();
    break;
  }
  if (!scalar(ingredients.at(0))) {
    raise << current_recipe_name() << ": first ingredient of 'wait-for-routine' should be a routine id generated by 'start-running', but got " << current_instruction().ingredients.at(0).original_string << '\n' << end();
    break;
  }
  if (ingredients.at(0).at(0) == Current_routine->id) {
    raise << current_recipe_name() << ": routine can't wait for itself! " << current_instruction().to_string() << '\n' << end();
    break;
  }
  Current_routine->state = WAITING;
  Current_routine->waiting_on_routine = ingredients.at(0).at(0);
  trace(Primitive_recipe_depth, "run") << "waiting for routine " << ingredients.at(0).at(0) << end();
  break;
}

:(before "End Scheduler State Transitions")
// Wake up any routines waiting for other routines to go to sleep.
// Important: this must come after the scheduler loop above giving routines
// waiting for locations to change a chance to wake up.
for (long long int i = 0; i < SIZE(Routines); ++i) {
  if (Routines.at(i)->state != WAITING) continue;
  if (!Routines.at(i)->waiting_on_routine) continue;
  long long int id = Routines.at(i)->waiting_on_routine;
  assert(id != Routines.at(i)->id);  // routine can't wait on itself
  for (long long int j = 0; j < SIZE(Routines); ++j) {
    if (Routines.at(j)->id == id && Routines.at(j)->state != RUNNING) {
      trace("schedule") << "waking up routine " <<style>pre { line-height: 125%; }
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